Effect of Strain on the Critical Current of Nb3Sn and NbTi Multifilamentary Composite Wires

Abstract

The critical currents of flexible Nb3Sn and NbTi composite wires have been observed to decrease as a function of strain. Characteristic samples of the data are presented along with a brief summary and intercomparison of the results for each wire type. In the NbTi wires the decrease commenced at strains of about 0.5%, but did not become appreciable (i.e. greater than ∼5%) until strains exceeded about 1.5%. The effect is almost totally reversible and the magnitude of the decrease is not strongly dependent on sample configuration or stabilization material. In the Nb3Sn composites the degradation in critical current is relatively much larger, becoming significant at strains ranging from 0.1% to 0.3% depending on the reinforcement technique used in the wires' construction. At high strains, the effect in Nb3Sn is only partially reversible. Also in the Nb3Sn composites, stress‐induced resistivities as high as 10−10 Ωcm were observed at currents well below Ic. This leads to significant Joule heating of the superconducting wire and ambiguities in the operational definition of “critical current”.